Cement compositions (hereafter "cement milks") have been mixed with cement modifying resin materials as cement modifiers, such as natural rubber, styrene-butadiene latex, acrylonitrile-butadiene latex, chloroprene latex, and the like, and have been in commercial use to improve the strength or the like of the concrete products obtained from such cement milks.
These cement modifiers are roughly classified into aqueous emulsion-type materials (hereafter "emulsion type cement modifiers"), and resin powdery materials (hereafter "powdery material type cement modifiers").
These cement modifiers are required to have the following basic properties:
1) Good mixing and dispersion stability when the cement modifier is stirred and mixed with cementitious material; PA0 2) Minimal change in viscosity when incorporated into a cement milk, with a good workability; PA0 3) Substantial improvement effect on physical properties such as the strength of the concrete products obtained by hardening the cement milk, such as surface hardness, compression strength, and flexural strength (hereafter, "strength"), and adhesion to a variety of substrates (hereafter "adhesion").
The extent of such improvement in strength, adhesion, and the like, of concrete products is, in general, said to depend on the glass transition temperature (hereafter "Tg") of the cement modifier and its minimum film forming temperature (hereafter "MFT").
Definition: The MFT is the temperature at or above which the polymer emulsion forms a continuous film. MFT is based on an apparatus based on the temperature gradient method described by Protzman and Brown (J. Appl. Polymer Sci 4, (1960)).
Apparatus for Measurement: An emulsion is filled into grooves, 15 mm wide, about 1 mm deep, and 400 mm long on a thick aluminum slab where a temperature gradient is maintained usually 20-80.degree. C., and is left standing for several hours. The aluminum slab has thermocouples embedded at about 25 mm intervals apart; as the water is allowed to gradually evaporate, a continuous film (or continuous films) begins to form starting from the high temperature end. The temperature at which a clear continuous film becomes discontinuous (formation of cracks and transportation into white powdery state) is recorded as the MFT.
A cement milk which uses an emulsion type cement modifier containing a high Tg polymer as an emulsion type cement modifier has a high MFT and requires a high temperature for hardening into a concrete product; in particular, when one casts during winter periods at temperatures lower than the MFT of the cement milk, a continuous film cannot be formed efficiently when a concrete article is made, hence, not only is it difficult to produce a concrete product retaining excellent strength, but there are sometimes adverse effects on the concrete products in other areas such as resistance to water, weathering resistance, resistance to acid, and the like (hereafter summarily called "durability") or in poor trowelability.
Consequently, it has heretofore been customary to improve the strength and adhesion of a concrete product by producing it from a cement milk containing an emulsion type cement modifier having a lower Tg polymer or a lower MFT.
From among these cement modifiers, powdery type cement modifiers are superior to emulsion type cement modifiers in that (1) their transport is easier in that it is not necessary to transport water as in the case of the emulsion type modifiers and they can be transported in bags, simplifying packaging; (2) they are easier to handle in that they do not suffer from freezing or phase separation as with the emulsion type modifier; (3) they give good workability and provide concrete products with no performance variations since a cement milk can be prepared by transporting a cement mixture obtained by premixing cementitious material with a cement modifier and simply adding water to the cement mixture at the work site to produce the cement milk.
However, a cement milk mixed with a powdery material obtained from a spray dried emulsion having a low Tg or low MFT as described above, if cast and hardened into a concrete product at low temperatures such as in a casting application in winter periods, cannot realize strength and adhesion at satisfactory levels; thus, low temperature-usable powdery material type cement modifiers have not yet been discovered.
Powdery type cement modifiers have been proposed, for example, a powdered cement modifier as described in U.S. Pat. No. 4,916,171, which is a spray dried powder of an emulsion containing core-shell structured acrylic polymer particles comprising a core polymer composed of an alkali insoluble emulsion polymer and a shell polymer composed of an alkali soluble emulsion polymer, part of which has been alkaline-neutralized.
The polymer which constitutes the shell of the core-shell structured acrylic polymer particles disclosed in U.S. Pat. No. 4,916,171 has a Tg of 100.degree. C. or higher and has a high acid value by virtue of having a large amount of MAA, as high as 20-50% by weight, copolymerized therein, where the polymer is neutralized to 80% or higher and has a molecular weight of 5000-50000 so as to achieve the dispersion stability of the polymer particles in the aqueous emulsion. However this product has been deficient in that the polymer constituting the shell of said polymer particles has the low molecular weight of 5000-50000, resulting in a concrete product which, after a cement milk containing such a polymer is hardened, is poor in durability, particularly in resistance to water, weathering resistance, and resistance to acid; production of polymer particles by spray drying such an emulsion using a spray drying process generates polymer particles which tend to block, whereby the blocked [caked] polymer particles are poor in friability, making them extremely difficult to handle.
In addressing these problems, U.S. Pat. No. 4,916,171 discloses a method of adding an anti-blocking agent such as inorganic particles like fine silica particles to the emulsion and spray drying, thereby attempting to prevent the resultant polymer particles from blocking, but polymer particles obtained by such a method end up containing a large amount of inorganic particles in the polymer particles, failing to fully exert advantageous properties as a cement modifier.
Polymer particles having such a high-acid-value shell polymer used as a cement modifier in a cement milk, will cause the viscosity to increase to a high level, reducing casting workability; in addition, concrete products obtained by hardening such a cement milk do not exhibit sufficient improvements in durability, strength, adhesion, and the like, so that such a polymer will not make a satisfactory cement modifier.
In addition, the polymer which constitutes the shell of these polymer particles a Tg as high as 100.degree. C. or higher so that a cement milk to which have been added said polymer particles of a high Tg shell polymer will show a high MFT, thereby showing inferior curing behavior and failing to provide the resultant concrete product with properties such as satisfactory durability, strength, adhesion, or the like, where such shortcomings are aggravated among other deficiencies when concrete products are produced, particularly at low temperatures as in winter.